(1) Field of the Invention
The present invention relates to a system for obtaining an optimum focusing position in an optical disc system, and further, relates to a focus servo control system for maintaining an optimum focusing position in an optical disc system during reading and writing operations, wherein the system for obtaining an optimum focusing position by an offset adjustment is incorporated.
Optical discs are used as large capacity external memories. The optical discs have a narrow (a few micrometer-width) track which is usually preformed spirally on its own surface.
Data is written on the track surface by impinging a high-intensity optical (light) beam onto the track surface to form a pit, and is read by impinging a low-intensity optical beam onto the track surface. Then, variations of the intensity of an optical beam reflected at the track surface caused by a pit are detected.
To carry out an effective and accurate writing and reading operation, the optical beam impinged onto the track surface is required to be focused in an optimum condition on the track surface.
(2) Description of the Related Art
FIG. 1 shows an outline of the construction of an optical disc apparatus.
In FIG. 1, reference numeral 1 denotes an optical disc, 11a denotes a rotation mechanism such as a motor, 11b denotes an rotating axle, 12 denotes a optical head, 13 denotes a track servo control portion, 14 denotes a focus servo control portion, 20 denotes a semiconductor laser device, 21a denotes a lens, 22 denotes a polarized beam splitter, 23 denotes a quarter wavelength plate, 24 denotes a mirror, 25 denotes an objective lens, 26 denotes a track actuator, 27 denotes a focus actuator, and 28 denotes a light sensing device.
The optical disc 1 is driven and rotates by the motor 11a through the rotating axle 11b. The optical head is moved in the radial direction by an actuating device (not shown) driven by a motor (not shown), and is then positioned to write or read data on an objective track of the optical disc 1.
In the optical head 12, light emitted from the semi-conductor device 20 is lead and focused through the lens 21a, beam splitter 22, quarter wavelength plate 23, mirror 24, and objective lens 25, and is then impinged onto the track surface of the optical disc 1. A light reflected by the track surface is lead through the objective lens 25, the mirror 24, the quarter wavelength plate 23, the beam splitter 22, and the lens 21b, to the light sensing device 28, and thus, is detected at the light sensing device 28.
Responding to the above detected signal, the light sensing device 28 generates an electric reflection signal, the amplitude of which corresponds to the intensity of the reflected and detected signal.
Generally, tracks are formed spirally or concentrically on optical discs at the pitch of a few micrometers, and the extension of a focused beam spot area on the track surface is less than one micrometer.
However, some eccentricity of an amount comparable with the above pitch or more may exist in the arrangement of the above tracks on optical discs. In addition, some waviness, which causes an off-focus at the track surface of optical discs, may exist in optical discs.
In spite of the above situation, the optical beam is required to be focused on the track surface, and the focused beam spot must follow the above narrow track. To fulfill the above requirement, the focus actuator (focus coil) 27, the focus servo control portion 14, the track actuator (track coil) 26, and the track servo control portion 13 are provided.
The focus actuator 27 moves the objective lens 25 in the optical head 12 in a direction perpendicular to the surface of the optical disc 1 to adjust the focus position of the impinging optical beam under the control of the focus servo control portion 14. The focus servo control portion 14 receives the output of the light sensing device 28, which consists of a plurality of receiving light signals, and generates a focus error signal (FES) to drive the focus actuator 27 in a feedback mode so that the optimum focus condition at the track surface of the optical disc 1 is maintained.
The track actuator 26 moves the objective lens 25 in the optical head 12 in the radial direction of the optical disc 1 to adjust the position of the impinging optical beam in the radial direction under the control of the track servo control portion 13. The track servo control portion 13 receives the output of the light sensing device 28, which consists of a plurality of receiving light signals, and generates a track error signal (TES) to drive the track actuator 26 in a feedback mode so that the position of the impinging optical beam is maintained at the center of the track width of the optical disc 1.
The principle of the focus servo control is explained with reference to FIGS. 2 to 4.
In FIG. 2, "f" denotes an on-focus condition wherein a focus position of an impinging light beam is just on the track surface of the optical disc 1, "f1" denotes an off-focus condition wherein a focus position of an impinging light beam is under the track surface of the optical disc 1, and "f2" denotes an off-focus condition wherein a focus position of an impinging light beam is above the track surface of the optical disc 1.
FIGS. 3A, 3B, and 3C show distributions of intensity of received light on a light input surface of the light sensing device 28. FIG. 3A shows the distribution in the above off-focus condition "f1", FIG. 3B shows the distribution in the above on-focus condition "f", and FIG. 3C shows the distribution in the above off-focus condition "f2".
The light sensing device 28 consists of four light sensing detectors. Each light sensing detector corresponds to one of four quadrants, each of which is denoted by "a", "b", "c", and "d", respectively.
The aforementioned focus error signal (FES) is defined as EQU FES=(a+b)-(c+d),
where "FES" denotes an intensity of the focus error signal (FES), "a" denotes an intensity of the light received by the light sensing detector "a", "b" denotes an intensity of the light received by the light sensing detector "b", "c" denotes an intensity of the light received by the light sensing detector "c", and "d" denotes an intensity of the light received by the light sensing detector "d", respectively. The focus error signal (FES) is obtained in the focus servo control portion 14.
FIG. 4 shows a variation of the amplitude of the above focus error signal (FES) as a function of the focus position regarding the track surface. Since the value of the focus error signal (FES) in the above on-focus condition "f" is zero, the focus servo control portion 14 controls the focus actuator 27 according to the above obtained value of the focus error signal (FES) so as to maintain the FES value near zero. Thereby the focusing position is maintained on the track surface in the resolution of a sub-micron order, even when waviness exists in the optical disc 1.
The principle of the track servo control is explained with reference to FIGS. 5 to 7.
In FIG. 5, "P" denotes a condition wherein a spot area of an impinging light beam is at the center of the track width of the optical disc 1. "P1" denotes an off-track condition wherein a spot area of an impinging light beam is in one side of the center of the track width of the optical disc 1. "P2" denotes an off-track condition wherein a spot area of an impinging light beam is in the other side of the center of the track width of the optical disc 1.
FIGS. 6A, 6B, and 6C show distributions of intensity of received light on a light input surface of the light sensing device 28. FIG. 6A shows the distribution in the above off-track condition "P1". FIG. 6B shows the distribution in the above condition "P". FIG. 6C shows the distribution in the above off-track condition "P2".
The aforementioned track error signal (TES) is defined as EQU TES=(a+d)-(c+b),
where "TES" denotes an intensity of the track error signal (TES), and the track error signal (TES) is obtained in the track servo control portion 13.
FIG. 7 shows a variation of the amplitude of the above track error signal (TES) as a function of the position of the spot area of the impinging light beam regarding the center of the track width. Since the value of the track error signal (TES) in the above on-track condition "P" is zero, the track servo control portion 13 controls the track actuator 26 according to the above obtained value of the track error signal (TES) so as to maintain the TES value near zero. Thereby the position of the spot area of the impinging light beam is maintained at the center of the track width in the optical disc 1, even when an eccentricity exists in the optical disc 1.
In an actual focus servo control system, however, the situation wherein the above value of the focus error signal (FES) obtained from the intensity of light detected at the light sensing device 28 is zero, does not necessarily correspond to the true on-focus condition wherein the focusing position is actually on the track surface of the optical disc 1, due to an off-center positioning of the light sensing device 28, or a level offset which arises in the internal circuit realizing the focus servo control system (portion 14). Therefore, it is necessary to adjust an offset value in an appropriate stage in the focus servo control portion 14.
To adjust the above offset value, a plurality of systems are proposed in the prior art.
U.S. Pat. No. 4,707,648 described the technique wherein the offset value which yields the maximum amplitude of the track error signal (TES) is obtained as the optimum offset value.
The measurement of the amplitude of the track error signal (TES) is carried out by rotating the optical disc without a track servo operation. A high frequency track error signal (TES) is required to be used in the measurement for obtaining a precise amplitude of the track error signal (TES). The high frequency of the track error signal (TES) means a large amount of eccentricity of the tracks on the optical disc because the variation of the level of the track error signal (TES) is caused when the impinging light beam traverses grooves, which are concave portions located between adjacent tracks as shown in FIG. 5. The phase difference of the light wave reflected at the groove from the phase of the light wave reflected at the track surface causes an interference between the light waves and decreases the intensity of the total reflected light.
However, recently, it is known that the optimum offset level obtained from the amplitude of the track error signal (TES) including the track error signal (TES) during the traverse of the grooves, does not necessarily give the actual optimum on-focus condition which enables the most precise operation of writing or reading data, i.e., the most precise operations of forming a pit on the track surface or detecting a pit on the track surface because the above traverse of the grooves shifts the optimum focusing position from the above actual optimum on-focus condition at the pits.
In another prior art technique, the offset value which yields the maximum direct current level of the intensity of the total reflected light from the surface of the optical disc, i.e., the output of the light sensing device 28, is obtained as the optimum offset value.
However, the optical system for impinging a light beam onto the track surface of the optical disc includes an astigmatic error due to a distortion of the optical components by temperature change or aging. Concretely, the astigmatism causes an elongated spot shape of the impinging light beam on the track surface of the optical disc. The above direct current level of the intensity of the total reflected light from the surface of the optical disc becomes a maximum when an elongated beam spot lies within the track surface area as shown in FIG. 8. Since data is written in the form of a series of pits, and is read by whether or not a pit exists at positions having predetermined (angular) intervals with each other, on a track, the beam spot elongated in the direction of the track may cause a serious error in writing or reading operations.